As the information-oriented society becomes more advanced, technologies for security protection of information processing systems have been developed. For example, ID cards are conventionally used to control access to computer rooms. However, the cards can be missing or stolen at high probability. To prevent this, introduction of personal recognition systems is starting, in which the fingerprint or the like of each person is registered in advance, and the fingerprint unique to each person is collated when he/she accesses the room, instead of using the ID cards.
Such a personal recognition system sometimes passes, e.g., a replica of a registered fingerprint. Hence, the personal recognition system must also recognize that the object is a living body in addition to fingerprint collation.
First Prior Art
The first prior art will be described which detects that an object is a living body (e.g., Japanese Patent Laid-Open Nos. 2001-12980 and 2001-62204). In a fingerprint detection device according to the first prior art, the absolute capacitance of a finger placed on the sensor surface is measured. For this purpose, using the sensor structure shown in FIGS. 22A to 22C, finger detection is done by a capacitive grid or capacitive plate which is arranged on the upper side of a finger sensor electrode.
A finger detection sensor electrode is electrically insulated from a fingerprint sensor electrode. Finger detection sensor electrodes may be arranged between fingerprint detection sensor electrodes 71 and on different surfaces, like finger detection sensor electrodes 72A shown in FIG. 22A. Alternatively, the finger detection sensor electrodes may replace the fingerprint detection sensor electrodes 71 and be arranged on the same surface, like a finger detection sensor electrodes 72B.
As shown in FIG. 22B, a finger detection sensor electrode 72C may be arranged on the upper side of the fingerprint detection sensor electrodes 71 while sandwiching a protective film 73A and covered with a protective film 73B. As shown in FIG. 22C, a finger detection sensor electrode 72D may be formed on the upper side of the fingerprint detection sensor electrodes 71 to be exposed through a protective film 73C.
On the basis of the finger capacitance thus measured, finger detection is done by the circuit arrangement shown in FIG. 23. The capacitance generated in a finger detection sensor electrode 81 is converted into a representative frequency by a representative frequency converter 82 and compared with a reference frequency or frequency range 83 by a frequency comparator 84 to determine whether the measured capacitance matches the predicted biological characteristic of living skin tissue. Accordingly, advanced finger detection is implemented.
Second Prior Art
The second prior art to detect that an object is a living body will be described (e.g., Japanese Patent Laid-Open No. 11-185020). In an individual authentication sensor according to the second prior art, a plurality of measurement electrodes 91 are arranged on a semiconductor substrate, and a common electrode 92 is arranged around the measuring electrodes 91, as shown in FIG. 24. Switching elements which selectively connect the respective measuring electrodes 91 to an I-V conversion circuit (detection circuit) 96 can be selected by scanning a row shift register 95 and column shift register 94.
A switch means 92A is arranged, which switches the common electrode 92 between the power supply and ground in measuring to detect contact of an authentication target to the measuring electrodes 91 or in standby without the measurement.
When the common electrode 92 is spaced apart from the measuring electrodes 91, the presence/absence of a characteristic feature of a living body can be detected between a fingertip and a body part except the finger and, for example, the back of the hand. More specifically, this technique uses the fact that the distance between the electrodes does not depend on the measurement result because the internal resistance of the finger is low.